A wedge issue is one that forces voters to choose sides, where the choice is a clear yes or no, stop or go, allow or ban.
In order to be successful the wedge has to apply to a hot button issue where a large group of voters are emotionally involved, and where the voters who cared most about the issue were formerly more likely to support your opponent. For example, gay marriage was used as a wedge issue by Republicans for years to divide blue collar (and otherwise Democratic) voters against themselves.

A wedge issue is often used to draw attention away from other weaknesses.

example:In 2004, George Bush made a tough line on terrorism his wedge issue, to draw attention away from his weak perfomance on domestic policy.